PS 10-116 - Eco-physiological factors influencing insect herbivory in Oregon White Oak (Quercus garryana)

Abstract: Oregon white oaks (Quercus garryana) provide unique habitats for diverse species along its native range in western North America. Land use changes over the past century have removed over 90 percent of this habitat and remaining habitat fragments are threatened by competition from conifer-dominated forests. As part of restoration efforts, we need to understand the ecological relationships that exist and may be restored. Although over 800 invertebrate species are known to be associated with Q. garryana, surprisingly little is known regarding the effects of herbivorous insects on oaks and how plant anatomical and physiological traits influence herbivory. To address these knowledge gaps, we studied patterns of herbivory in Q. garryana in three distinct habitats: savannah dominated by grasses with scattered oaks, closed canopy forest where conifers overtop oaks, and a restored woodland where conifers were logged in 2018-2019. We sought to relate patterns in herbivory to habitat type, restoration status, and tree and leaf anatomical and physiological traits. In 2019 we sampled leaves from 20 Q. garryana across the three habitat types, and this sampling was repeated in 2020 with a larger sample-size. Midday and pre-dawn leaf water potential and trunk diameter at breast height were measured in the field. Insect herbivory, specific leaf area, trichome density, percent nitrogen, C:N ratio, and d13C were measured from the collected leaves, and linear mixed-effects models were developed to determine which factors best predicted herbivory damage.

In 2019, trees in the savannah habitat tended to have higher levels of herbivory than the forest and woodland habitats. This trend was consistent with a significant positive relationship between herbivory damage and more negative midday leaf water potential, as trees in the savannah were more water-stressed in late summer than trees in other habitats. There was also a significant negative relationship between leaf trichome density and herbivory damage. The statistical model incorporating both trichome density and midday water potential explained 73% of the variation in leaf herbivory, suggesting that trees experiencing greater water stress and with less dense trichomes were subjected to the greatest amount of herbivory by insects. Other leaf and tree physiological and anatomical traits did not significantly predict herbivory. Analysis of 2020 leaves is ongoing but preliminary analysis shows consistency in water-stress between habitats. Our current results demonstrate the importance of eco-physiological factors such as drought and plant responses to it in order to accurately predict herbivory levels in this foundational species.